[JessicaAnne]

Hi,

trying to do a small experiment, I observed something strange...

in a glass tube, I put some MnCl2 solution that I wanted to be oxidized. I tried several way of oxidation to show people. I attached a O2 tube to the glass tube and tied them together with a tie-wrap (made out of nylon). Then Mn2+ got oxidized to MnO2 easily, but what was more surprising is that the tie-wrap just died in seconds. I replaced it many times, but after a few seconds, it would fall by itself.
Then nylon has a lot of double bond, so something must attack them quite efficiently. Could it be radicals, ozone? Any idea? Thanks

[JessicaAnne]

Oh and I forgot to say, there was pressure applied by the gas in the solution tube as I was controlling the oxygen out (just by pinching the tube with my fingers and that was the reason I needed tie-wraps) and I heated the system smoothly with a Bunsen Burner...

I read that hydroperoxyl could also be the tie wrap killer, but I wouldn't understand why, like ozone, it would be produced here...

[JessicaAnne]

The tie wrap wasn't in contact with the solution. It was just to be sure that the gas tube would stay in place.
The solution was acidic but putting a tie wrap in a solution sample didn't do a thing.
Moreover, I tied some of them around the experiment, in random places, and they all eventually broke by themselves. The white nylon having cracks, like used rubber on it. Which let me think it is gaseous...

[BluRay]

Cl^- in acid solution can be oxidized to Cl₂ from oxigen, so maybe it's chlorine. Just an idea.

[JessicaAnne]

Thanks BluRay,

I kinda thought about that because Nylon is indeed very sensitive to Cl2, but then I am still puzzled by what could have oxydized Mn2+?
Standard potential necessary to oxidize Mn2+ is around 1.5V and oxygen standard potential is only around 1.2v. That's why I thought it was ozone which oxidation potential is 0.5V higher than Mn2+...

[BluRay]

Thanks BluRay,

I kinda thought about that because Nylon is indeed very sensitive to Cl2, but then I am still puzzled by what could have oxydized Mn2+?
Standard potential necessary to oxidize Mn2+ is around 1.5V and oxygen standard potential is only around 1.2v. That's why I thought it was ozone which oxidation potential is 0.5V higher than Mn2+...

You are talking about the potential to oxidize Mn²⁺ to MnO₄^-, not to MnO₂. The last is 1.22V, in acidic solution (1M). The potential of O₂ --> H₂O is instead 1.23, in acidic solution, so it's greater: oxygen in acidic solution can easily oxidize Mn²⁺ to MnO₂.
By the way: I remember now that a simple way to generate Cl₂ gas is to add HCl to MnO₂...

[JessicaAnne]

Thanks,

I think I am going to work that out knowing quantitatively what's inside the solution... Although Cl₂ would be a nice way to understand the observed reactions -as Cl₂ could indeed oxidize easily Mn²⁺ along with O₂^-, by quickly looking at the electrochemical potentials, it sounds odd that O₂ would react with HCl to give Cl₂

O₂ + 4H⁺ + 4e  2H₂O    E⁰=1.23V

4Cl^-  2Cl₂ + 4e   E⁰=-1.36V

the overall reaction gives a negative potential, meaning the reaction should go the other way round.

Now you say that a simple way to generate Cl₂ gas is to add HCl to MnO₂ and that is done in classrooms as I read it. But looking at the potentials, we have the same a priori. Meaning the reaction:

MnO₂(s)+4HCl(aq)  Cl₂(g)+MnCl₂(aq)+H₂O(l)  has a  :delta:(E⁰) negative.

Coming from the following half reactions:
Cl₂(g) + 2 e    2 Cl⁻     E⁰=+1.36

MnO₂(s) + 4 H⁺  + 2 e  Mn²⁺ + 2 H₂O       E⁰=+1.23


So I guess it may be a matter of concentrations... Or am I missing something?

[BluRay]

Yes, I think it's a matter of concentrations and of pH, the first reaction you wrote, that is the reduction of oxygen, is strongly favoured by low pH; for what concerns
MnO₂(s) + 4HCl(aq)    Cl₂(g) + MnCl₂(aq)+H₂O(l)
it's also matter of physical states: chlorine is gaseous and can leave the solution, displacing the reaction to the right (of course with HCl here we intend a water solution of HCl).

[JessicaAnne]

Thanks BluRay, I think you got my mystery solved